1. Field of the Invention
The present invention relates to a method for producing a superconducting article such as a superconducting wire. More particularly, it relates to a novel method for producing a superconducting article comprising a core body composed of an alloy and a surface layer composed of a superconducting material supported or surrounding on the core body.
2. Description of the Related Art
Under the superconducting condition, the electric resistance becomes zero and perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power.
Realization of superconducting coils which permit generation of a very high magnetic field is expected to accelerate development in the field of fusion power generation as well as in the field of MHD power generation or motor-generators. The development of superconducting coils is also in demand other industrial fields such as electric power reservation system; transportation systems such as magnetic levitation trains or magnetically propelling ships; medical appliances such as a high-energy beam radiation unit; or scientific instruments such as NMR.
In addition to the abovementioned electric power applications, the superconducting materials can be used in the field of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. A tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as high sensitive sensors or detectors for sensing very weak magnetic fields, microwaves, radiant rays or the like since variation of electromagnetic waves or magnetic fields is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demand in the field of high-speed computers in which the power consumption per unit area is reaching the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
However, the critical temperature could not exceed 23.2 K. of Nb.sub.3 Ge which was the highest Tc for the past ten years.
The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 dicloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10 K. and hence usage of liquidized helium (boiling point of 4.2 K.) as cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30 K., which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature in the order of 90 K. in February 1987. Still other types of new superconducting materials which were reported recently are a compound oxide of Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system which exhibit such high Tc as more than 100 K. and which are chemically much stable than the abovementioned YBCO type compound oxide or the like.
Hence, the possibility of an existence of high-temperature superconductors have burst onto the scene.
However, these new type superconducting materials are ceramic materials of compound oxides which do not possess such a high plasticity as well-known metal type superconducting materials such as Ni-Ti alloy. In fact, they are produced by a sintering technique in which a compact of powder material is sintered in a furnace. However, the sintered articles are fragile and easily brake even under a very weak mechanical stress, so that they can not or are difficult to be shaped or deformed into a desired shape such as a wire by the conventional plastic deformation techniques. In order to realize a reliable and practical superconducting structure, it is indispensable that the structure possesses enough strength and tenacity which is sufficient to endure bending force during usage.
Taking the abovementioned situation into consideration, the present inventors have proposed a process for manufacturing a ceramic wire by the steps comprising filling a metal pipe with a material powder of ceramic, performing plastic deformation of the metal pipe filled with the ceramic metal powder to reduce the cross section of the metal pipe, and then subjecting the deformed metal pipe to heat-treatment to sinter the ceramic material powder in the metal pipe in the United States of America patent application Ser. No. 152,713 and Ser. No. 161,480 which are commonly assigned and now abandoned, Ser. No. 182,489 and Ser. No. 189,366, now U.S. Pat. No. 5,122,507.
These solutions are themselves satisfactory but the present inventors have continued to develop another process which can produce the superconductor of compound oxide more simply and completed the present invention.
Therefore, an object of the present invention is to provide a new and simple method for manufacturing a superconductor.